The present invention relates generally to inspection systems. More specifically, it relates to light collection mechanisms for inspecting semiconductor wafers and other types of patterned samples.
Conventional darkfield optical inspection tools locate defects on patterned wafers by scanning the surface of the wafer with a tightly focused laser spot and measuring the amount of light scattered by the illuminated spot on the wafer. Dissimilarities in the scattering intensity between similar locations in adjacent dies are recorded as potential defect sites.
The dynamic range of this optical scattering is typically substantial. Changes in scattering intensity of more than a million to one within a single die are not uncommon. This high dynamic range is intrinsic to the optical configuration of the instrument and the scattering properties of the wafers and defects of interest. Because this dynamic range is substantially greater than the reliable measurement range of existing instruments, inspection operators are forced to accept an unpleasant compromise between inspecting with too low a sensitivity in some portions of the die, and temporarily overloading the instrument's detection electronics in other regions.
In general, scanning the wafer with the smallest possible laser spot size maximizes sensitivity to defects by maximizing the spatial resolution of the scattering image. However, this increased resolution generally correlates with an increased pixel density within the light collectors or detectors to properly sample the image. The detectors typically include a sensor for detecting the scattered light and generating an analog signal based on such detected light and an analog-to-digital converter (ADC) for converting the analog detected signal into a digital detected signal. The digital detected signal may then be analyzed for defects. Since all the pixels are measured serially, and only a limited amount of time is available to scan each wafer, there is a fundamental relationship between the speed of the measurement electronics and the maximizing of the spatial resolution of the scattering image. To enable high spatial resolution, higher bandwidth analog electronics and faster ADC's are often utilized.
In addition to maximizing the speed of the measurement electronics to thereby maximize spatial resolution, it is desirable to maximize the dynamic range of the light that is discernable by the measurement electronics. However, there is a fundamental trade-off in ADC's between speed and dynamic range. That is, dynamic range is typically limited by noise and offset errors, both of which tend to increase with speed.
Accordingly, there is a need for improved inspection mechanisms that are capable of quickly detecting light having a relatively high dynamic range.